DC Servomotor circuit having drive current controlled as a function of motor speed

Abstract

A drive circuit is provided for controlling the energization of a DC motor as a function of speed to maintain the motor speed equal to a speed defined by a speed command signal. The speed command signal is summed with a velocity feedback signal to produce a speed error signal which is used as an input to a motor drive circuit. In accordance with the invention, the speed error signal is limited in order to continuously limit the current through the motor to a continuously changing maximum depending on the speed of the motor. During an initial part of the speed range, the limiting level is increased to maintain a constant maximum motor current. During an intermediate speed range, the limiting level decreases so as to sharply decrease current with increasing motor speed; and during a final speed range, the limiting level increases so as to gradually decrease current with increasing motor speed.

What is claimed is: 
     
       1. An apparatus for predetermining the maximum current supplied by a drive circuit to a bidirectional DC motor coupled to a tachometer providing a tachometer signal having a magnitude representing the actual motor speed and a polarity representing the actual direction of motor rotation, said drive circuit being controlled by the magnitude and polarity of a bipolar speed error signal generated by a comparator in response to the algebraic difference between the tachometer signal and velocity command signal, said command signal having a magnitude representing the desired motor speed and a polarity representing the desired direction of motor rotation, the apparatus comprising a bipolar limiting means having an input responsive to the tachometer signal and an output connected between the comparator and the drive circuit for limiting the magnitude of the speed error signal to positive and negative limiting levels for both polarities of the tachometer signal, said limiting means causing the magnitude of the limiting level to be (1) varied continuously in accordance with a plurality of linear functions, and in response to the tachometer and command signals having polarities representing the same direction of motor rotation, each linear function controlling over a portion of the full speed range of the motor to produce a nearly exponential profile of motor current, and   (2) directly proportional to the tachometer signal in response to the polarity of the tachometer signal representing a direction of motor rotation opposite the direction of motor rotation being represented by the polarity of the command signal, whereby the maximum current available to the motor is predetermined exclusively by the speed error signal over the full speed range of the motor.   
     
     
       2. The apparatus of claim 1 wherein the means for limiting the speed error signal further comprises: (a) means for increasing the limiting level continuously to produce constant motor current over an initial speed range of the motor;   (b) means for reducing the limiting level continuously to produce rapidly decreasing motor current over an intermediate speed range of the motor; and   (c) means for increasing the limiting level continuously to produce gradually decreasing motor current over a final speed range, the motor current decrease being at a rate less than the rate of decrease over the intermediate motor speed range.   
     
     
       3. The apparatus of claim 2 wherein the means for limiting the speed error signal includes, for each of the positive and negative limiting levels, a respective circuit comprising: (a) a semiconductor device having a control terminal for controlling the conductivity of a conduction path through the device, said device being connected between a line carrying the speed error signal and a point of reference potential;   (b) means for applying the tachometer signal to the control terminal with a polarity to increase the magnitude of the limiting level established on the speed error signal line as motor speed increases over the initial motor speed range;   (c) a bias circuit also connected to the control terminal; and   (d) a bias control circuit responsive to the tachometer signal for controlling the bias circuit to reduce the bias on the control terminal over the intermediate motor speed range to an extent overcoming the action of the tachometer signal on the control terminal, thereby reducing the magnitude of the limiting level established on the speed error signal line over the intermediate motor speed range.   
     
     
       4. The apparatus of claim 3 wherein the means for limiting the speed error signal further includes means connected to the bias control circuits for preventing further reduction of the bias taking place beyond the intermediate part of the motor speed range whereby in a final motor speed range the limiting level is again increased by an increase in the tachometer signal. 
     
     
       5. The apparatus of claim 4 wherein the preventing means further comprises a controlled current shunt circuit connected to the control terminal of the semiconductor devices and responsive to the tachometer signal, said shunt circuit being conductive during the final motor speed range to cause the rate of increase of the limiting level over the final motor speed range to be less than the rate of increase of the limiting level over the initial motor speed range. 
     
     
       6. The apparatus of claim 5 wherein the means for limiting the speed error signal further comprises a bias potential source connected to each bias circuit for reducing in proportion to the magnitude of the tachometer signal, the bias potential for the one of the semiconductor devices which is active when the polarities of the tachometer signal and the command signal represent opposite directions of motor rotation.